This invention concerns an audio signal processing system including an audio signal volume control network arranged to suppress the audible effects of transients produced at the moment the system is energized or de-energized.
An audio signal processing system often includes provision for controlling the amplitude of audio signals processed by the system, to thereby control the volume of audio information reproduced by a loudspeaker associated with the system. In many signal processing systems, the audio signal amplitude can be conveniently controlled by means of an electronic signal attenuator (e.g., a gain controlled amplifier) which responds to a variable DC gain control (i.e., volume control) voltage. One such electronic attenuator is incorporated in the TDA-2791 integrated circuit commercially available from Philips Corporation. The volume control voltage is commonly derived from a user adjustable volume control potentiometer, and is applied to a gain control input of the electronic attenuator for varying the signal gain of the electronic attenuator and thereby the volume level of a reproduced audio signal, in accordance with the setting of the volume control.
In practice, a filter capacitor is often coupled to the signal path which couples the variable DC gain control voltage to the gain control input of the electronic attenuator. Such capacitor may be required, for example, to remove ripple components which may be present in the DC operating supply voltage associated with the volume control potentiometer, to suppress spurious signals such as noise, and to control the slew rate (rise time) of the volume control voltage when the system is initially energized. In some systems (e.g., including the TDA-2791 integrated circuit or a similar circuit), the electronic attenuator comprises a differential amplifier responsive to the audio signals and to gain control signals, applied to a differential input. Operating supply and bias voltages derived from the system power supply are provided to the differential amplifier, and an operating potential also derived from the system power supply is applied to the volume control potentiometer.
In an audio signal processing system of this type, it is desirable to suppress the audible effects of operating supply voltage transients which can occur when the system is energized and de-energized, respectively causing the operating supply and bias voltages to be applied to and removed from the differentially gain controlled amplifier and the volume control potentiometer. Such transients can cause the amplitude of the output signal from the gain controlled amplifier to increase momentarily with a magnitude and sense for producing an annoying audible "thump" from the sound reproducing loudspeaker of the system. The audible effects of transients associated with energization and de-energization of the system are suppressed by apparatus arranged according to the present invention.